The present invention relates to an optical scanning device for use in an image forming apparatus, and a scanning lens therefor.
In an optical scanning device for an optical printer, digital copier or similar image forming apparatus, a polarizer polarizes, at a uniform angular velocity, a beam issuing from a light source and forming a line image elongate in a direction corresponding to a main scanning direction. A scanning lens converges the polarized beam to a desired surface to be scanned in the form of a beam spot. The scanning device scans the desired surface at a uniform velocity with the beam spot. The direction corresponding to the main scanning direction refers to a direction parallel to the main scanning direction at any position on a virtual optical path which is formed by developing linearly an optical path between the light source and the desired surface along the optical axis. Likewise, a direction corresponding to a subscanning direction refers to a direction parallel to a subscanning direction at any position on the above virtual path.
It is a common practice with the scanning device to use a simple scanning lens in order to reduce the size and cost of the device. A simple scanning lens is taught in, e.g., Japanese Patent Laid-Open Publication No. 7-174998. An image forming lens for scanning is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 4-50908.
To prevent the scanning lens from increasing in size, it is preferable that the lens be positioned as close to the polarizer as possible. This, however, brings about a problem that the magnification of the lens increases. For example, if the position where the line image is formed is even slightly deviated on the virtual optical path due to the slight positional error of optics for forming the line image in the optical axis direction, the focusing position of the beam spot is noticeably shifted. As a result, field curvatures are noticeably deteriorated, compared to designed field curvatures. This is particularly true with the field curvature in the subscanning direction.
While the scanning lens functions to cause the polarized beam to converge to the desired surface in the direction corresponding to the subscanning direction, it must implement uniform velocity scanning in the direction corresponding to the main scanning direction. To realize uniform velocity scanning and desirable field curvatures with a simple lens, it is necessary that at least one surface of the lens has a special shape different from an arc in the direction corresponding to the main scanning direction. Generally, the scanning lens is implemented as a convex lens in order to converge the polarized beam to the desired surface in the direction corresponding to the main scanning direction. For this reason, the lens is often formed by molding plastics. However, the problem is that when the lens is provided with a biconvex configuration, the thickness is apt to increase in the paraxial portion and noticeably differs from the paraxial portion to the peripheral portion. Consequently, the lens implemented as a molding of plastics often suffers from sinking, waving and other errors in configuration. The lens with such errors has critically low performance in practice although the designed performance may be optimal. This problem is particularly serious when a wide-angle lens is desired to render the scanning device compact or to extend the scanning range, or when a lens is located in the vicinity of the polarizer in order to reduce the size of the lens.
When use is made of a simple lens, uniform velocity scanning capability will be effectively promoted if the surface of the lens facing the polarizer is provided with a convex shape. In the case, however, the thickness of the lens decreases with a decrease in the radius of curvature, aggravating the difference in thickness between the paraxial portion and the peripheral portion. Moreover, if the radius of curvature of the surface facing the polarizer is small, the angle at which the polarized beam is incident to the above surface, i.e., the angle between the outward normal of the surface and the chief ray of the incident beam increases in the direction corresponding to the main scanning direction. This is likely to deteriorate the field curvatures and uniform scanning due to the errors in the configuration of the above surface.
In addition, the scanning device should preferably have a compact and broad effective main scanning width. The polarizer therefore must have a broad effective polarization angle.